Helping Out a High-Temperature Superconductor

“In theory, superconducting materials can conduct an enormous amount of electric current. But when incorporated into actual devices, certain factors tend to limit the current,” said Brookhaven materials scientist Qiang Li. “We studied these factors and found that one, which we call ‘substrate roughness,’ can actually significantly increase the current-carrying capacity.”

The superconducting material studied here consists of the elements yttrium, barium, copper, and oxygen. Dubbed YBCO, it is a member of a class of copper- and oxygen-containing superconductors called “cuprates.” Cuprates are “high-temperature” superconductors because they superconduct at temperatures much “warmer” than conventional superconductors — for example, -300°F rather than -440°F. This difference, while not huge, is enough to make cuprates more viable for practical applications than materials that must be kept much colder.

The researchers deposited a YBCO layer onto a substrate prepared with two distinct areas: a rough, corrugated region with nanometer sized ridges and grooves, and a smooth region. This configuration allowed the group to directly compare the behaviour of the YBCO film on both surface types. They were able to do this using electrical-transport measurement techniques, which track the amount of supercurrent passing through the material, and “magneto-optical” imaging, a technique used to study superconductors by following their magnetic behaviour.

“What we found is remarkable and surprising,” said lead author Zuxin Ye, a graduate student under Li’s supervision. “Rather than limiting the current, the nanoscaled corrugated surface produces more than a 30 percent increase in the supercurrent carried by the YBCO films. This suggests that metal substrates with some degree of roughness at the nanoscale might help improve the performance of high-temperature superconductors.”